As a personal computer, video equipment, and a digital camera are becoming popular, a projection image display device (hereinafter referred to as a projector) has been often used for displaying moving or still image.
Generally, in a projector, light fed from a light source is collected at a predetermined place and modulated by light modulation devices, such as a DMD (digital mirror device) and a liquid crystal element, by which color images are displayed on the screen. To display images with high luminance on a large screen, the projector requires a high-intensity light source. From the reason, a high-intensity discharge lamp has been mainly employed for the light source. In recent years, however, to meet the demands for lower power consumption, a longer lasting light source, and a compact body of the device, some suggestions have been made. In such suggestions, light-emitting elements, such as a light-emitting diode and a laser diode, are used and a plurality of light sources is employed for increase in intensity (for example, see patent literature 1). In a device where a light-emitting element is used for obtaining a high-intensity light source, high durability of the light source is essential. In particular, in a case where light from a laser light source excites phosphors for light emission, deterioration of phosphor layers exposed to excitation light often invites poor durability of a projector.
To address the problems above, prior art technique has introduced some suggestions on climate control of a laser light source and decrease in energy of excitation light onto phosphors (for example, see patent literatures 2 through 4).
According to the methods, proper control of temperature, humidity, and energy of excitation light protects a light source and phosphors from aging deterioration.
However, when the projector is used in various situations, it is difficult to maintain the temperatures of the light source and the phosphors at an optimum level according to ambient temperature. A change in temperatures is unavoidable. In the conventional technique, there is no examination on suppressing deterioration of a light source and phosphors according to temperature change of them. The phosphors have a large influence on durability of the projector, and the light-emitting efficiency of them depends on temperatures. When the phosphors have change in temperature while being exposed to excitation light, relative output intensity of the phosphors changes, as shown in FIG. 14. The conventional methods have no measures against changes in light-emitting output according to changes in temperatures of the phosphor layers exposed to excitation light. Therefore, the light-emitting output of the phosphors cannot be maintained at a proper output level as the light source of the projector. This invites deterioration of phosphors, resulting in degradation of image quality (i.e. decrease in brightness of the screen).